Field of Invention
The invention relates to a method for comprehensively treating low-concentration ammonia-nitrogen wastewater by completely recycling, and particularly relates to a method for recycling ammonia from low-concentration ammonium chloride wastewater, which pertains to the field of comprehensively treating ammonia-nitrogen wastewater.
Description of Related Arts
China is a country being abundant in resources of rare earth, with a rare-earth yield of about 150,000 tons per year, and the low-concentration ammonium chloride wastewater is a discharge liquid inevitably generated in the non-ferrous industry, and especially in the rare-earth wet separation industry. According to statistics, the average consumption of the ammonia water or ammonium bicarbonate for separating one ton rare-earth oxide is greater than 4 tons, and as limited by the production process, the content of ammonium chloride in the discharged wastewater is relatively low (the concentration thereof is mostly 0.35-2 mol/L), and even so the amount of ammonium chloride discharged per year in the rare-earth industry exceeds 1,000,000 tons. Furthermore, synthesizing 1,000,000 tons ammonia requires 800,000,000 cubic meters natural gas, or requires about 300,000 tons anthracite, and thus if the ammonia is not recycled and reused from the wastewater, wasting of resources may be caused. On the other hand, the low-concentration ammonium chloride wastewater may cause serious environment pollution. The discharge of ammonia-nitrogen wastewater in the rare-earth industry across Ganzhou region of Jiangsu province, China had caused a serious pollution event previously, and thus the ammonia-nitrogen content of the Taihu Lake area exceeded a certain limit. In the Baotou area, as the flow of Yellow River was reduced each winter, the seriously excessive ammonia-nitrogen wastewater caused a serious pollution. The environmental protection department of People's Republic of China had pay great attention to this and thus immediately ordered the rare-earth industry to conduct rectification and reformation. Therefore, it is an important technical issue currently plaguing the rare-earth industry how to recycle and reuse the low-concentration ammonia-nitrogen wastewater.
Currently, a commonly-used process for recycling the low-concentration ammonia-nitrogen wastewater mainly is recycling the ammonium chloride by directly crystallizing through evaporative concentration, or recycling hydrochloric acid and ammonia water by evaporating the ammonium chloride together with a circulating agent, wherein the former process is disadvantageous in the following aspects: the direct evaporation causes a great thermal energy consumption, the environmental protection and control operate with money loss, and since the market area of ammonium chloride is affected by a localized selling radius, a large amount of ammonium chloride is piled up and unsalable, and thereby many enterprises have to discontinue the recycling production. The later process is disadvantageous in that concentrations of the produced hydrochloric acid and ammonia water are too low to be used, the process has great power consumption, the cost for operating with money loss is very high, and thereby industrialized production is unable to be conducted.